Differential signaling is generally a technique of transmitting information electrically by means of two complementary signals sent on two separate wires. Often the technique can be used for both analog signaling, as in some audio systems, and digital signaling, for example as in RS-422, RS-485, twisted-pair Ethernet, PCI Express and USB. The opposite technique, which is more common but lacks some of the benefits of differential signaling, is called single-ended signaling.
In the electronics industry, and particularly in portable and mobile devices, there is a continuing tendency to lower the supply voltage in order to save power and reduce unwanted emitted radiation. A low supply voltage, however, frequently causes problems with signaling because it reduces the noise immunity. Differential signaling generally helps to reduce these problems because, for a given supply voltage, it gives twice the noise immunity of a single-ended system.
Low-voltage differential signaling (LVDS) includes a differential signaling system. LVDS frequently uses this difference in voltage between the two wires to encode the information. In general, the transmitter injects a small current (e.g. 3.5 mA) into one wire or the other, depending on the logic level to be sent. The current passes through a resistor, which is often matched to the characteristic impedance of the cable, at the receiving end, then returns in the opposite direction along the other wire. From Ohm's law, the voltage difference may be determined. The receiver frequently senses the polarity of this voltage to determine the logic level. This type of signaling is often referred to as a current loop.
In general, the small amplitude of the signal and the tight electric- and magnetic-field coupling between the two wires reduces the amount of radiated electromagnetic noise and power lost to resistance of the conductors.